Inter-Switch Link (ISL) trunking is not supported.
Note
Private VLANs
Private VLANs provide traffic separation and security at the Layer 2 level.
A private VLAN is one or more pairs of a primary VLAN and a secondary VLAN, all with the same primary
VLAN. The two types of secondary VLANs are isolated and community VLANs. Hosts on isolated VLANs
communicate only with hosts in the primary VLAN. Hosts in a community VLAN can communicate only
among themselves and with hosts in the primary VLAN but not with hosts in isolated VLANs or in other
community VLANs.
Regardless of the combination of isolated and community secondary VLANs, all interfaces within the primary
VLAN comprise one Layer 2 domain, and therefore, require only one IP subnet.
Spanning Tree
This section discusses the implementation of the Spanning Tree Protocol (STP). Spanning tree is used to refer
to IEEE 802.1w and IEEE 802.1s. When the IEEE 802.1D Spanning Tree Protocol is referred to in the
publication, 802.1D is stated specifically.
STP Overview
STP provides a loop-free network at the Layer 2 level. Layer 2 LAN ports send and receive STP frames,
which are called Bridge Protocol Data Units (BPDUs), at regular intervals. Network devices do not forward
these frames but use the frames to construct a loop-free path.
802.1D is the original standard for STP, and many improvements have enhanced the basic loop-free STP.
You can create a separate loop-free path for each VLAN, which is named Per VLAN Spanning Tree (PVST+).
Additionally, the entire standard was reworked to make the loop-free convergence process faster to keep up
with the faster equipment. This STP standard with faster convergence is the 802.1w standard, which is known
as Rapid Spanning Tree (RSTP).
Finally, the 802.1s standard, Multiple Spanning Trees (MST), allows you to map multiple VLANs into a
single spanning tree instance. Each instance runs an independent spanning tree topology.
Although the software can interoperate with legacy 802.1D systems, the device runs Rapid PVST+ and MST.
You can use either Rapid PVST+ or MST in a given VDC; you cannot mix both in one VDC. Rapid PVST+
is the default STP protocol.
STP L2 loop occurs when a Cisco Nexus 5000 switch in a vPC receives a frame with MACSEC Ethertype.
The switch in the vPC blocks one of its ports and the packet is forwarded even though there is an STP block
and thus creating a loop. This is a known limitation. To resolve this, troubleshoot the STP topology; identify
the STP port on the blocking switch; and perform a shut/no shut on the port. This helps to break the loop.
Cisco NX-OS uses the extended system ID and MAC address reduction; you cannot disable these features.
Note
Cisco Nexus 5500 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
2
Overview
Private VLANs
